In February 2003 a Sudanese militia began targeting tribes in Darfur, a region in western Sudan. The militia killed and displaced vast numbers of people in what would later be called genocide. Early surveys by the World Health Organization (WHO) found a two-month death rate of 10,000 a month, and later estimates simply extended the death toll based on that rate, up to 180,000 after 18 months. In the spring of 2005, however, the U.S. Department of State reported its own figure, including a lower estimate of 63,000 to 146,000. Some news organizations still cite the lower number, stating that tens of thousands have died.

In an attempt to form a more accurate assessment, sociologists calculated death rates and total deaths during a 19-month period using what they consider the seven best primary surveys from camps in the state of West Darfur. Together the surveys, conducted by the WHO and the humanitarian group M¿decins Sans Fronti¿res, document pre-camp violence in five camps and in-camp mortality throughout the state. Projecting their data to 31 months, or about three quarters of the conflict's duration, they estimated that between 58,000 and 85,000 died in West Darfur alone. Assuming the same ratios of death and displacement in adjoining North and South Darfur, they arrive at a conservative estimate of 170,000 to 255,000 deaths.

The death toll is likely much higher, notes John Hagan, co-author of the report published in the September 15 Science. Their reported upper limit "likely increases to the 400,000 range if the further year of the conflict is estimated and if missing and presumed dead persons are included," he says. Other experts agree that the new tally is a low estimate. This study should "not in any way bill itself, or be billed as, a global mortality study of the Darfur genocide," says Sudan researcher Eric Reeves of Smith College. A major survey of violent mortality counted a death toll of 397,000 as of April 2005, he observes. Better counting of the dead will not be possible until investigators can safely enter the region.